The present invention pertains to a method for the continuous filtration of fluids containing solids, in particular the treatment of waste water, by means of a filter bed, consisting of granular filter media which is moved continuously through a vertically oriented cylindrical, annular space (2), through which the fluid to be filtered flows continuously in a radial direction, whereby the granular filter media containing the retained solids is withdrawn continuously at the bottom of said annular space, transported to the top of the filter, cleaned from the filtered solids, and then returned back to the annular space at its upper end, whereby:
a granular filter media having two or more particle size distributions, ranging from coarse to fine, is being used, and this granular media is being classified before and during its introduction to the annular space in such a way that the particle sizes in the fill material of the filter bed decrease in the flow direction of the fluid to be filtered.
In methods of the prior art (see hereto DE-C-329 176, DE-A-1 932 221, or EP-B-0 128 234), it is typical that the fluid to be filtered flows through the annular space containing the filter bed in a radial direction from the outside to the inside, and the grain sizes of the filter media decrease from the outside to the inside.
A problem with such filter methods consists in the fact that the fluids to be filtered often contain solid particles in form of flocs, which can clog the pores between the granular media rather quickly due to the low flow velocities. The danger for such clogging exists primarily in the filter bed near the fluid inflow area (surface filtration).
Such clogging even takes place when the grain size of the filter media is larger in the entry zone to the filter than in the exit zone.
To counter such clogging by solid flocs, it is known in the art (see hereto EP 0291 538B) that a fine mesh screen can be provided before the fluid entry point which retains the flocs. The disadvantage is that this mesh screen must be cleaned from the deposited material in regular intervals, which requires considerable cleaning efforts.
It is the objective of the present invention to develop the method described above further, such that fluids containing solid flocs can be filtered more readily. In particular, clogging through the deposition of separated solid flocs shall be avoided by better and simpler means. At the same time, a continuous withdrawal means for the separated solid flocs is provided.
To solve this task, the present invention proposes, based on the method referenced above, to decrease the grain sizes in the fill in the filter bed in radial direction from the inside to the outside, and also to pass the fluid to be filtered through the filter bed in a radial direction from the inside to the outside with a resulting decrease of the flow velocity.
Surprisingly, it has been found that by reversing the flow direction, as proposed in the present invention, the clogging described above can be completely avoided. For the first time, in the present invention due consideration is given to the significant changes in velocities for a radial flow through a cylindrical annular space. In a radial flow through a cylindrical annular space, the velocity decreases significantly along the flow path in a greater than proportional way from the inside to the outside, because of the geometric conditions. In the method of the present invention, the flow velocity of the fluid to be filtered is highest in the entry zone to the filter bed, which is where the coarse granular material is located. As a result we find a deep filtration in this zone, whereby the entrained solid flocs are transported deep into the filter bed, which can accept much greater quantities of flocculated solids until it clogs. During the subsequent separation of the finer waste particles from the fluid to be filtered in the area of finer grained filter media, the flow velocity is significantly reduced, which allows for the finer waste particles to deposit on the fine-grained filter media. As a result, the risk of clogging is almost completely eliminated using the method of the present invention, and the efficiency of the filter plant is significantly improved.
A useful further development in the method of the present invention intends to have the fluid to be filtered undergo, in the area of the outer wall of the annular space containing the filter bed, an additional filtration through a fine-meshed sieve or through filter nozzles. This additional filtration step at the end of the flow path of the fluid to be filtered reliably retains the fine grained filter media and also offers a high flow resistance, such that a pressure gradient in the filter bed develops, which contributes to even out the flow pattern in the filter bed in the axial direction.
Furthermore, it is planned to separate all larger and heavier solid particles from the fluid to be filtered by passing the influent through a deflection chamber, where an 180xc2x0 reversal takes place. This very simple and very effective pre-separation means for larger and heavier particles also reduces the amount of solid particles entering the filter media and thus the risk of clogging.
A further object of the invention is to provide a means to filter continuously fluids laden with solids in order to implement the above method having the following characteristics:
a cylindrical container having a vertically oriented cylindrical annular space containing the granular filter media, and having inner and outer walls which are permeable for fluids,
a withdrawal means, located at the lower end of the annular space containing the granular media loaded with solids,
an airlift whose central pipe extends from the withdrawal means to the upper end of the annular space,
a cleaning means for the granular filter media, located between the top discharge point of the airlift pipe and the upper end of the annular space,
an adding means, located between the cleaning means and the upper end of the annular space, in order to return the cleaned granular filter media back to the annular space in the vicinity of its outer wall,
a conduit for the fluid to be filtered, located between the permeable inner wall and the airlift pipe, having at its upper end a connection for the fluid to be filtered, and being in connection at its lower end with a deflection chamber, in which the fluid to be filtered is reversed from a descending into a rising flow direction, and being directed into the annular gap formed by the conduit and the permeable inner wall, whereby the deflection chamber is equipped with a sludge withdrawal means which is in connection with the suction head of the airlift.
The sludge settling out in the deflection chamber is added in the conical area outside the filter zone to the airlift, and thus does not increase the loading on the filter bed. Further advantages result from the initially descending flow direction of the fluid to be filtered, the reversal into a rising flow direction and the inflow into the filter bed out of the rising flow direction. When taking the fluid to be filtered out of a rising flow, more solids are being introduced into the lower portion, i.e. the area where the granular filter media is soon to be withdrawn anyway, than in the upper portion. This is due to the fact that in the rising flow a certain de-mixing of the fluid to be filtered takes place.
Furthermore, it is intended to cover the permeable outer wall of the receiving space on its inner side with a micro filtration cloth, which rests upon a support structure with drainage channels. This micro filtration cloth is being cleaned continuously at its inner surface through the continuously descending filter bed. This cleaning can be intensified through hydraulic surges; such surges provide a short-term flow reversal through the micro filtration cloth. The support structure, upon which the micro filtration cloth rests, guarantees that the filtrate behind the filter cloth can be removed continuously and without any problems. This support structure can be in the form of corrugated metal or have similar drainage properties, for example, in the form of egg cartons or mushrooms.
In addition, the form of the drainage support allows to increase the area of the micro filter cloth. A level controlled discharge valve is compensating the increase in pressure drop due to the micro filter cloth. This level controlled discharge valve regulates the discharge in such a way that behind the micro filter cloth on the filtrate side (clean water side) a negative pressure of up to 9.5 m WC (13.7 psig) is generated, but without causing a negative pressure in the filter bed.
Alternatively, the permeable outer wall can also be equipped with filter nozzles having micro slits in a star-shaped arrangement inside wall openings. Also the micro slits of these filter nozzles are continuously kept free from contaminants due to the moving granular filter media. The filter nozzles have an excellent service life.
Finally, it is planned to design the cleaning means as a hydraulic classifier with a steady water inflow, and equipped with a fine bubble aeration system. In this hydraulic classifier the generally lighter settled out solids are separated from the heavier granular particles of the filter media and transported upstream. The fine bubble aeration is effected by means of a porous membrane located near the bottom of the hydraulic classifier. The rising air bubbles activate and intensify the cleaning process, which reduces the required amount of water by about half.